Method And Device For Separating Air By Cryogenic Distillation

ABSTRACT

A method and an apparatus for distilling air by cryogenic distillation is presented. The invention relates to an apparatus using a double air separation column comprising a medium pressure column and a low pressure column connected thermally together by a reboiler-condenser.

The invention relates to a method and an apparatus for distilling air bycryogenic distillation. In particular, it relates to an apparatus usinga double air separation column comprising a medium pressure column and alow pressure column connected thermally together by areboiler-condenser.

EP-A-1078212 describes an apparatus where a heat exchanger is positionedbelow the low pressure column, this being placed beside the mediumpressure column.

According to the present invention, an air separation apparatus isprovided comprising a main exchange line, a first element with anelongated shape comprising the medium pressure column of a doublecolumn, surmounted by a reboiler-condenser and an auxiliary columncontaining at least one theoretical plate constituting the lower part ofthe low pressure column, a second element with an elongated shapeconstituting the remainder of the low pressure column, means forconveying air from the main exchange line to the medium pressure column,means for conveying a head gas from the medium pressure column to thereboiler-condenser, means for conveying the condensed head gas from thereboiler to the medium pressure column, means for conveying a head gasfrom the auxiliary column to the vessel of the second element with anelongated shape, means for conveying a vessel liquid from the secondelement with an elongated shape into the head of the auxiliary column, asub-cooler placed below the first or second element with an elongatedshape, means for conveying a vessel liquid from the medium pressurecolumn to the sub-cooler and then to the second element with anelongated shape and/or means for conveying a head liquid from the mediumpressure column to the sub-cooler and then to the second element with anelongated shape, and means for conveying a head gas from the secondelement with an elongated shape to the sub-cooler.

Preferably, the sub-cooler is placed below the second element with anelongated shape.

The sub-cooler and the second element with an elongated shape may beplaced in a pack of which the length does not exceed 35 metres, or even30 metres.

This configuration makes it possible to best occupy the volume of thecold box by:

balancing the heights of the two column sections,

placing the sub-cooler intelligently in order to optimise the crosssection of the cold box,

leaving the medium pressure column on the ground permitting a diphasicair feed charged on the column,

ensuring the satisfactory charging of liquid on the pump for raisingliquid,

making it possible to have a cold box including a parallel section andminaret with a height less than 35 m, or even 30 m while usingstructured corrugated-crossed packings.

The invention will be described in greater detail with reference to thedrawing that shows an air separation apparatus according to theinvention.

An air separation apparatus comprises a first element with an elongatedshape A and a second element with an elongated shape B. The firstelement with an elongated shape A comprises the medium pressure column 1of a double column surmounted by a reboiler condenser 5. An auxiliarycolumn is located above this reboiler containing a few theoreticalplates of structured corrugated-crossed packings, constituting the lowerpart 2 of the low pressure column.

The second element with an elongated shape B is composed of theremainder of the low pressure column 3 also containing structuredcorrugated-crossed packings.

A sub-cooler 6 is located below the second element.

The apparatus includes means 11 for conveying purified compressed andcooled air in the main exchange line, to the medium pressure column.Nitrogen from the head of the column is conveyed to the reboiler 5 whereit is condensed.

Gaseous oxygen 17 is withdrawn from the vessel of the auxiliary column.

An oxygen-rich gas 15 is withdrawn from the head of the auxiliary columnand conveyed to the vessel of the column 3. A vessel liquid 13 from thecolumn 3 is conveyed to the head of the auxiliary column 2 after havingbeen pressurised in a pump P.

The first element with an elongated shape is placed at ground level. Thesecond element with an elongated shape is raised up by means of a metalframe, this frame being itself placed at ground level. This frame servesat the same time to support the cooler. The pump P is also positioned atground level.

In this way, the heads of the auxiliary column 2 and the second elementwith an elongated shape are situated substantially at the same level.

The rich liquid from the vessel of the medium pressure column is cooledin the sub-cooler before being conveyed to the second element with anelongated shape.

Similarly, one or more nitrogen-rich liquids coming from the head of themedium pressure column may be cooled there before being conveyed to thehead of the second element with an elongated shape.

At least one head gas from the second element with an elongated shape isreheated in the sub-cooler before being conveyed to the main exchangeline.

1. An apparatus for separating air comprising a main exchange line, afirst element with an elongated shape (A) comprising the medium pressurecolumn (1) of a double column, surmounted by a reboiler-condenser (5)and an auxiliary column (2) containing at least one theoretical plateconstituting the lower part of the low pressure column, a second elementwith an elongated shape (B) constituting the remainder of the lowpressure column, means (11) for conveying air from the main exchangeline to the medium pressure column, means for conveying a head gas fromthe medium pressure column to the reboiler-condenser, means forconveying the condensed head gas from the reboiler to the mediumpressure column, means for conveying a head gas (15) from the auxiliarycolumn to the vessel of the second element with an elongated shape,means for conveying a vessel liquid (13) from the second element with anelongated shape into the head of the auxiliary column, a sub-cooler (6)placed below the first or second element with an elongated shape, meansfor conveying a vessel liquid from the medium pressure column to thesub-cooler and then to the second element with an elongated shape and/ormeans for conveying a head liquid from the medium pressure column to thesub-cooler and then to the second element with an elongated shape, andmeans for conveying a head gas from the second element of elongatedshape to the sub-cooler.
 2. The apparatus as claimed in claim 1, whereinthe sub-cooler (6) is placed below the second element with an elongatedshape (B).
 3. The apparatus as claimed in claim 2, wherein thesub-cooler (6) and the second element with an elongated shape (B) areplaced in a pack of which the length does not exceed 35 meters, or even30 meters.